Patent GB-308796-A (GB)
Title: Improvements in or relating to high frequency telephone systems
IPC: H04M7/16
Filed: 1929-03-18
Abstract: 308,796. Associated Telephone & Telegraph Co., (Assignees of Nelson, M. L., and Hershey, H. E.). March 30, 1928, [Convention date]. Automatic exchange systems.-In automatic exchange systems using power lines and modulated high-frequency carrier current for interexchange working, the transmission of selective and signalling currents over the power lines is controlled by automatic switches responsive to impulses. The exchanges are arranged to transmit and receive on one of a number of carrier frequencies, and the receiving apparatus at a called exchange controls its transmitter so that it replies on the frequency used by the calling exchange. The system is applicable for working between two groups of exchanges, the groups being associated with different power lines, and each group working on an individual frequency for internal calls. The power lines are interlinked. In the system described, each exchange comprises a connector T1, Fig. 1, of the Strowger type to which local subscribers have direct access and which is also used to control the setting up of inter-exchange calls over the carrier channels. For this purpose, certain sets of line terminals in the banks of the switch are left unconnected, the first two digits, in the case of inter-exchange, calls serving to prepare the high-frequency equipment and select the frequency to be used in accordance with the setting of the control wipers P1, P2. Four further digits are then repeated by the connector line relay A to the carrier channel selected, the first two for setting a connector at the distant exchange and the remaining two for code ringing the wanted party. The transmitter, Fig. 3, comprises an oscillator 78 controlled by one or other of two variometers 84, 85 according to the frequency selected, and a modulator 70, the modulated carrier current being fed by a transformer 72 into conductors 40, 41 leading to the power line. The oscillator 78 is controlled by a valve 77, as described later, so that it is in operative condition only when signals are being transmitted. The outgoing speaking path to the modulator comprises conductors 1, 2, Fig. 1, hybrid coil T, Fig. 4, and amplifying valves 63, 68, Fig. 3. The receiving arrangements at each exchange comprise two receivers, Figs. 4 and 5, connected to the power line over conductors 35, 36, and tuned to different frequencies. A connector T2 or T3, Fig. 2, is taken into use in accordance with the receiver which responds to the incoming signals, prepares the local high frequency equipment, and selects the proper frequency for communicating with the calling exchange. The connector is set by incoming impulses, rings the wanted party and then releases. The corresponding receiver and connector at other exchanges also respond, but the corresponding terminals of the connectors are either left blank, or they are connected up to subscribers, the wanted party being selected by code ringing. The wanted party, on replying, is connected to conductors 1, 2 leaving to the hybrid coil T, incoming speech currents from the receiver being fed to this coil over conductors 31, 32. The input circuits of the receivers are shunted by valves 109, 154 normally biassed to form high resistance paths, the bias being varied automatically during transmitting periods to make the shunts effective to disable the receivers. The relay group A - - K, Fig. 1, controls the highfrequency circuits both for incoming and outgoing calls in conjunction with the connectors T1, T2, T3. As will be subsequently explained, valve 110, Fig. 4, is used to time the operation of relay B, Fig. 1, after relay C has been operated. The two frequencies used for transmitting will be referred to as the " A " and " B " frequencies. Local calls. It is assumed that subscriber No. 77 calls subscriber No. 86. The operation of calling subscriber's line and cut-off relays, corresponding to L, M, connects his line to relay A1 of the connector T1. Relay B.1 holds the cut-off relay which by operating relay A prevents any other Subscriber from operating his line relay. The first digit is repeated over relay C1 to the vertical magnet F1. The second digit is repeated to rotary magnet H1 and relay D1 in parallel, and when D1 falls back at the end of the train, switching relay E1 (upper) energizes over wiper P2 in series with the wanted subscriber's cut-off relay M. Relay E1 locks over its lower winding, and relay M, owing to an interlock between its armature and that of L, only pulls up sufficiently to open the circuit of L. Relay E1 also completes a circuit over wiper P1 for relay D which completes the speaking connections at the connector and opens the circuit of relay H so that this relay and also relay K which, as will appear later, have been previously operated to start up generators for the high-frequency equipment, fall back to prevent high-frequency interference with the local connection. The calling party now dials the code ring consisting of digits 2, 0, which are repeated to sluggish relay D1 so that ringing current from generator 93. Fig. 3, is connected up for two periods, short and long. When the called party replies, he receives feed current from relay E, release being under the control of the calling party. Outgoing call by subscriber No. 86 using A " frequency carrier current. The subscriber is connected to the connector T1 as in the case of a local call. The cut-off relay M operates relay A which connects 125 volts positive to the alternator 87, Fig. 3, and also energizes relay H which operates slow-to-pull-up relay K to connect 6 volts positive to the filament of valve 109, Fig. 4, and start a generator, not shown, for supplying current to the highfrequency equipment. The line relay A1 at the connector energizes relay C which opens the connection between the grid-biassing battery GB, Fig. 4, and the grid of the valve 110, the plate of which is connected, to 125 volts positive over the winding of relay B. The grid potential of valve 110 rises since it is connected, over a resistance to 6 volts negative., and the subsequent gradual increase in the plate current operates relay B which releases if at any time C releases and applies additional negative grid bias. The calling party now dials two digits corresponding to spare line-terminals in the bank of T1 to prepare the out-going high-frequency circuit. The connector responds as in a local call and at the first vertical step, relay F operates over ON11 and releases C so that B falls back. When D1 releases at the end of the second digit, switching relay E1 (upper) energizes over wiper P2 since battery is connected over resistances to contacts corresponding to spare line-terminals. The corresponding terminals in the bank of wiper P1 for preparatory digits for an " A " frequency call are unconnected. Relay E1 locks up over its lower winding, completes, a circuit for tone alternator 87, Fig. 3, and transformer winding 65 in series, and makes the circuit of C in. dependent of F. The calling party now dials the two digits of the wanted subscriber's number and the code ringing digits, the impulses being repeated by A1 which opens the circuit of the alternator 87 to interrupt the modulation oF the carrier current a corresponding number of times. The preparatory digits were not effective in this manner since the alternator circuit was open at a contact of El. The impulses induced in the secondary winding 66 are impressed on the grid of the amplifying valve 68 which is coupled by a transformer to the modulating valve 70. The oscillator 78 is tuned by the variometer 84 and feeds an amplifier 79 the output of which is modulated by impulses in the plate circuit of the valve 70, so that modulated carrier current passes over conductors 40, 41 to the distant, exchange. The plate-filament circuit of the oscillator 78 is in series with the platefilament circuit of control valve 77 which, when the exchange is not actually transmitting, is biassed so as to block the oscillator. When transmission commences, voltage taken from the transformer secondary 66 is impressed on the grid of amplifier 75 which varies the bias of valve 77 so as to render the oscillator operative. During the trains of impulses, the circuit of relay C is interrupted by relay A1 so that this relay remains unoperated, but when all the digits have been sent out, this relay pulls up and is followed by B when the plate current of valve 110 has built up sufficiently (say after three seconds). The speaking path is closed and the signalling path opened by relay B. The speaking path passes over conductors 1, 2 to hybrid coil T, Fig. 4, and thence over conductors 33, 34 to the input transformer 61 of amplifier 63 having a primary winding in its output circuit coupled to a secondary 64 which is connected, in a closed non-metallic circuit over a contact of relay B, with a primary winding of the input transformer Z of ampifier 68 from which the path e.xtends as described for the signalling impulses. The operation of the receiving apparatus and automatic switches at the distant exchange will be understood from subsequent description of incoming calls. When the calling party hangs up, relays A1, B1 restore and connector T1 releases. Relays M, A, H and K fall back in turn. Outgoing call using " B " frequency current. Variometer 84 at the oscillator is replaced by variometer 85 by dialling suitable preparatory digits such as 4, 5 to bring wiper P1 of the connector into engagement with terminal 45 So that when switching relay E1 energizes, ground is connected over P1 to relay G, which operates relay S, Fig. 3, to switch over to variometer 85 and change the balancing network by substituting condensers 82 for condensers 81. Incoming call using A " frequency current. After the subscriber at the distant exchange has dialled the preparatory digits to set in operation the carrier equipment at his exchange, four dig